The present invention relates to a magnetron for use in microwave application apparatuses, such as microwave ovens.
A magnetron serving as an electron tube generating microwaves has a relatively high oscillation efficiency and delivers high output with ease. Hence, the magnetron is widely used as a microwave generator for microwave application apparatuses, such as microwave ovens.
A conventional magnetron will be described below.
FIG. 13 is a sectional view showing a conventional magnetron for use in general microwave ovens. As shown in FIG. 13, a cathode portion 250 is disposed at the central portion of the magnetron, and an anode portion 260 is disposed around the cathode portion 250. The cathode portion 250 comprises a filament 201, and a center lead 204 and a side lead 205 connected to the filament 201 via end hats 202 and 203, respectively, provided on both ends of the filament 201. The anode portion 260 comprises a cylindrical anode 206 and a plurality of vanes 207. The vanes 207 are disposed so as to project from the inner circumferential face of the anode 206 to the filament 201 placed at the center and so as to maintain a predetermined distance between the ends of the vanes 207 and the filament 201.
A pair of magnetic poles 209 and 210, having a similar conical shape, is disposed so as to face each other at both ends of the anode 206 in the axial direction of the cylinder. In FIG. 13, an input portion 211 for supplying electric power to be applied to the filament and for supplying high voltage for driving the magnetron is provided outside the lower magnetic pole 210 in the axial direction of the cylinder. An output portion 212 for transmitting and emitting microwaves is provided outside the upper magnetic pole 209 in the axial direction of the cylinder. The cathode portion 250, the anode portion 260, the magnetic poles 209 and 210, the input portion 211 and the output portion 212 constitute the main body portion of the magnetron.
Furthermore, the conventional magnetron is provided with a pair of ring-shaped permanent magnets 213 and 214. One magnetic pole face of the permanent magnet 213 or 214 is coupled to the magnetic pole 209 or 210. The other magnetic pole face is magnetically coupled to a U-shaped frame yoke 215 or 216 made of a ferromagnetic material. The magnetic circuit configured as described above supplies a magnetic field to an electron motion space 217 formed between the vanes 207 and the filament 201. One end of an antenna lead 218 for outputting microwaves is connected to one of the vanes 207 of the anode portion 260. The other end of the antenna lead 218 is guided outside and connected to the output portion 212.
The conventional magnetron delivering an microwave output power of approximately 1 kW has the following specifications and dimensions. The oscillation frequency of the magnetron is in the 2,450 MHz band. The number of the vanes 207 is 10. The diameter φa of the inscribed circle formed by the cathode-side ends of the vanes 207 is 9.0 mm. The outside diameter φc of the coil-shaped filament 201 is 3.9 mm. The height H of the vanes 207 is 9.5 mm in the axial direction of the cylinder, and the thickness T of the vanes 207 is 2.0 mm. The gap G between the cathode-side ends of the adjacent vanes 207 is 0.9 mm. The ratio of the gap G and the thickness T of the vanes 207 is G/(G+T)=0.31. The magnetic flux density at the electron motion space 217 was 0.195±0.010 teslas when measured on the center lead 204 at the central portion between the pair of magnetic poles 209 and 210.
In the conventional magnetron having the above-mentioned configuration, electrons are emitted from the filament 201 to the vanes 207 by heating the filament 201 and by applying a predetermined voltage across the cathode portion 250 and the anode portion 260. The electrons are rotated around the filament 201 by a magnetic field inside the electron motion space 217, thereby generating microwave energy. This microwave energy is transmitted to the output portion 212 by the antenna lead 218 electrically connected to one of the vanes 207. The microwave energy is emitted to the inside of a microwave oven or the like, for example. The oscillation efficiency of the magnetron at this time is calculated from the DC input (anode voltage×anode current) applied across the cathode portion 250 and the anode portion 260 and from the measured value of the microwave power emitted from the output portion 212. In a typical conventional magnetron, an oscillation efficiency of 74.1% was obtained by outputting a microwave power of approximately 1 kW at an anode voltage of 4.5 kV and an anode current of 300 mA.
The oscillation efficiency of the magnetron is determined by the product of electron efficiency, i.e., the motion efficiency of electrons, and the circuit efficiency relating to circuit constants, such as Joule loss and dielectric loss. In other words, the oscillation efficiency η is represented by electron efficiency ηe×circuit efficiency η c.
It is known that the electron efficiency ηe is represented with respect to the anode voltage by the following equation (1), and that the electron efficiency ηe is enhanced by raising the anode voltage.ηe=1−mV2/2e V a  (1)a. (ηe: electron efficiency, m: electron mass, V: electron orbital velocity, e: electron charge, Va: anode voltage)
From another point of view, it is known that the electron efficiency ηe is represented with respect to the magnetic flux density by the following equation (2), and that the electron efficiency ηe is enhanced by raising the magnetic flux density.
                                                                                          η                  e                                =                                  1                  -                                                            (                                              1                        +                        σ                                            )                                                                                                                                            B                            ⁡                                                          (                                                              1                                -                                σ                                                            )                                                                                ⁢                          N                                                                          0.7144                          ⁢                          f                                                                    -                                              (                                                  1                          -                          σ                                                )                                                                                                                                                                    σ                =                                                                                                    (                                                  ϕ                          ⁢                                                                                                          ⁢                                                      a                            /                            2                                                                          )                                            2                                        -                                                                  (                                                  ϕ                          ⁢                                                                                                          ⁢                                                      c                            /                            2                                                                          )                                            2                                                                                                  B                      ⁡                                              (                                                  1                          -                          σ                                                )                                                              ⁢                    N                                                                                      }                            (        2        )            b. (ηe: electron efficiency, B: magnetic flux density, f: oscillation frequency, N: number of vanes, φa: diameter of inscribed circle at cathode-side ends of vanes, φc: outside diameter of coil-shaped filament)
In order to meet the needs for world-wide energy conservation in recent years, the oscillation efficiency η of the electron is required to be enhanced. Hence, improvement in the oscillation efficiency of the magnetron has become necessary. In the conventional magnetron, the oscillation efficiency is enhanced by increasing the density of the magnetic flux supplied to the electron motion space and by raising the anode voltage. However, in order to raise the anode voltage, the power source for driving the magnetron must be replaced with a power source for high voltage, and the dielectric withstand voltages of the magnetron and its peripheral components must be raised. As a result, improving the oscillation efficiency of the conventional magnetron leads to cost increase.
Furthermore, in the conventional magnetron, it is necessary to use large ring-shaped permanent magnets in order to increase the density of the magnetic flux supplied to the electron motion space. Because of this upsizing of the ring-shaped permanent magnets, the size of the magnetron itself required to be large. This causes a problem wherein the magnetron is not compatible with already available products and also causes a problem wherein the serviceability of the magnetron becomes low during repair or the like.
Still further, when a ring-shaped permanent magnet that was expanded in its diametric direction and thus flattened so as to be made larger is placed once in a low-temperature environment of −40° C. or less, for example, during the air shipment of the magnetron, the ring-shaped permanent magnet has an irreversible demagnetization characteristic. This causes a problem of demagnetization. As a result, in the conventional magnetron placed once in the low-temperature environment of −40° C. or less, the density of the magnetic flux in the electron motion space lowers to a predetermined value or less, thereby causing a problem of lowering the oscillation efficiency of the magnetron.